Korea is one of the most complex, critical, and yet understudied of Japan's foreign policy relationships. While much attention in US policy and academic circles has focused on Japan's future relations with China as the key variable for regional stability in the twenty first century, an integral part of the security dynamic in East Asia has been driven by the Japan–Korea axis. In the late-nineteenth century and early twentieth century, two major power wars in Asia (i.e., Sino-Japanese and Russo-Japanese) had this relationship as a proximate cause. During the cold war, the Japan–Republic of Korea (ROK) axis facilitated the American presence as an Asia-Pacific power and security guarantor. And in the post-cold war era, outcomes in the Japan–Korea (united or still divided) relationship are critical to the shape of future balance of power dynamics in the region and with it, the future American security presence. How then should we be thinking about future Japanese relations with the Korean peninsula? What are Tokyo's hopes and concerns with regard to Korea? How do they view the prospect of a united Korea? Is there a Japanese ‘grand strategy’ regarding the peninsula?